Process for preparing a foaming slag former, product and use thereof

ABSTRACT

Process for preparing a foaming slag former for electric furnaces comprising the steps of aggregating solid slag particles into a coarser granular material and carbonating the solid slag particles to form the foaming slag former. The solid slag particles are preferably aggregated before carbonization, so that the carbonates form a solid matrix binding the particles together.

This application is a national stage entry of PCT Application No.PCT/EP2009/050445 filed Jan. 15, 2009, and claims the benefit of PCTApplication No. PCT/EP2008/050412 filed Jan. 15, 2008, which is herebyincorporated by reference herein in its entirety.

The present invention relates to a process for preparing a foaming slagformer for electric arc furnaces with at least 20 wt. % of solid slagparticles, to the product of this process, and to its use in electricarc furnaces.

The use of electric arc furnaces is well-known in steelmaking as well asfor the production of non-ferrous alloys. Typically, such an electricarc furnace comprises a refractory-lined vessel, and, within saidvessel, a set of graphite electrodes. In such electric arc furnaces thecharge is usually introduced at the beginning of a process cycle andmelted down by an electric arc between the electrodes. Slag formers areusually added so as to create a layer of slag floating on the melt so asto protect the melt from oxidation. Additionally, the slag also acts asa thermal blanket, reducing heat losses, and helps protect therefractory lining from radiation from the electrodes.

Those slag formers usually comprise calcium and magnesium oxides.

To increase the thermal insulation and refractory lining protectionprovided by the slag, it has become common to use additives to foam saidslag. Usually, these additives are carbon sources, such as coke, which,under the conditions of the hot furnace, produce carbon monoxide bubblesin the slag. It has been proposed, for instance in French patentapplication publication FR-A-2 634 787, Japanese patent applicationpublication JP 62-023920, and U.S. Pat. Nos. 5,395,420 and 6,375,711 touse calcium and/or magnesium carbonates, that is, limestone, dolomiteand magnesite, as both slag formers and carbon dioxide sources in themolten slag so as to foam it. This has however encountered the drawbackthat these natural carbonates are normally obtained from limestone,dolomitic limestone, dolomite and magnesite, raw materials of increasingcost.

At the other end of the process cycle, after discharging the electricarc furnace, the solid slag is a waste material. The disposal of thatwaste is a significant problem, in particular for slag containingpollutants such as heavy metals, e.g. chromium, nickel, or molybdenum,and/or halogens, e.g. fluorine, which could constitute a significantenvironmental and health hazard, should they leach out into theenvironment, and in particular into water supplies and/or the foodchain. Moreover, this solid slag may still contain significant amountsof metal, which is thus lost to the production process. In particular,it is difficult to extract the finest metal inclusions from the solidslag. Furthermore, recycling the resulting fine metal dust byreintroducing it into the electric arc furnace has a low yield, as muchof this fine dust is simply blown out of the furnace by the strongupdraft.

It has been proposed by M. Guzzon et al in “Recycling of ladle slag inthe EAF: improvement of the foaming behaviour and decrease of theenvironmental impact”, 2006 ATS International Steelmaking Conference,Paris, France, Dec. 14-15, 2006, to introduce ladle slag powder into anelectric arc furnace so as to recycle the slag powder and to therebyalso improve at the same time the foaming behaviour. This effect isexplained in the article by the introduction of dicalcium silicateparticles which form suspended second-phase particles in the molten slagacting as CO nucleation sites, which leads to a high amount offavourable gas bubbles in the foamy molten slag.

However, this prior art still presents significant drawbacks.

Because of the β-γ transition in such dicalcium-silicate-containingladle slag, much of the slag is in a powdery form which hinders itsintroduction into the EAF. Apart from the obvious problem of blow-off ofsuch fine particles by the strong updraft in these furnaces, these fineparticles also tend to clog up chutes and trough lances.

The objectives of the present invention are therefore those of providingboth a means of safe disposal of the solid slag, in particular its fineor powdery fraction, and a foaming slag former with reduced consumptionof raw materials, including metal and/or metal ore and carbonatesources. In particular, the present invention addresses the problem ofproducing a slag former with better flowability and general handling.

These objectives are fulfilled by the steps of aggregating solid slagparticles to form a granular material formed by coarser grains andcarbonating them to produce said slag former. These coarser grains havemuch increased flowability with respect to the initial slag particles,which may be handled, transported and blown into an electric arc furnacewithout clogging up chutes, containers or ducts, or being blown off bythe updraft. Aggregating the particles into coarser grains alsoincreases the homogeneity of the product and thus its safety andeffectiveness in use.

Preferably, said aggregation step may be performed before saidcarbonation step, so that, after the carbonation step, a solid matrixcomprising carbonates formed during the carbonation step binds the slagparticles together within each grain. This matrix produces a hardgranular material which is particularly easy to handle withoutnecessarily requiring any binding additive. Moreover, it may form acrust around the grain which prevents water absorption, thus furtherincreasing the safety of the granular material as a foaming slag former.

While the recycling of ladle slag disclosed in the abovementioned priorart article of M. Guzzon et al. increases the amount of nucleation sitesand thus produces a finer, more distributed foam with a smaller bubblesize, it does not significantly contribute to the volume of gas releasedin the molten slag, for the following reasons:

The ladle slag can absorb a lot of water, which presents a clearexplosion hazard during their introduction in the EAF, as any waterabsorbed by the particles instantly vaporises. To prevent this, in thedisclosed prior art method, the ladle slag powder is used while stillfresh, before any significant hydration of the calcium and magnesiumoxides, never mind carbonation of the resulting hydroxides, can takeplace. The natural hydration of calcium and magnesium oxides, anintermediate step for the carbonation, is in fact explicitly discouragedin said prior art article. In the disclosed prior art method, the ladleslag is thus substantially free of carbonates.

It is thus a further objective of the present invention to increase thevolume of gas released in the molten slag to form foam. To this purpose,at least 2 wt. % more preferably at least 3 wt. % of carbonates(expressed as CO₃ ²⁻) are produced during the carbonation step (measuredon the basis of the total dry weight of the foaming slag former). Forinstance, said carbonation may be carried out with a gas, such as forexample a flue gas, comprising at least 5 vol. %, preferably at least 8vol. % and more preferably at least 10 vol. % of carbon dioxide.

By the carbonation step, calcium and magnesium oxides/hydroxides presentin the solid slag particles are homogeneously converted into calcium andmagnesium carbonates, which in the furnace will decompose into calciumand magnesium oxides and foam-inducing carbon dioxide. The exothermiccarbonation in an atmosphere with a high content of carbon dioxide alsodries the slag and reduces its water absorption, significantlyincreasing the safety of its use as a foaming slag former. By recyclingthe solid slag into a highly carbonated slag former, the consumption ofcostly raw materials, such as limestone, dolomitic limestone, dolomite,magnesite, scrap metal and/or metal ore, is reduced, all the whilereducing the problem of disposing with a potentially hazardous waste.This slag former is also sulphur-poor, which makes it comparable to thehighest-quality natural carbonate sources.

Carbonating with industrial flue gases moreover provides an advantageoususe to carbon dioxide that normally would otherwise be directly emittedinto the atmosphere as a greenhouse gas. It must be noted that thiscarbon dioxide is thus continuously recycled, rather than permanentlystored, as proposed by C. H. Rawlins et al in “Steelmaking slag as apermanent sequestration sink for carbon dioxide”, pp. 25,26,28, SteelTimes International October 2006, or by S. Goto in “Eco-Materials MadeFrom Industrial By-Products and Carbon Dioxide”, Water Dynamics: 4thInternational Workshop on Water Dynamics.

Advantageously, said carbonation step may be performed using a gascomprising less than 30 vol. %, preferably less than 25 vol. % and morepreferably less than 20 vol. % of carbon dioxide. This allows the use ofindustrial flue gases without any particular treatment for thiscarbonation step.

Advantageously, said carbonation step may be performed at a temperatureof between 10 and 100° C., in particular at a temperature lower than 80°C., and more particularly at a temperature lower than 60° C. Again, thisallows the use of industrial flue gases without any significant additionof thermal energy.

Advantageously, said carbonation step may be performed at a pressurelower than 10 bar and preferably substantially at ambient pressure. Notonly this dispenses with potentially expensive high-pressure equipment,but the comparatively slow carbonation at these pressures hassurprisingly been found to produce a granular material with hardergrains. Presumably, the low pressure promotes crystal growth instead ofcrystal nucleation, this latter being known to give rise to very smallcarbonate crystals characterised by poor binding properties.

Preferably, after said carbonation said gas may still be used to reducethe alkalinity of effluent waters having a pH higher than 11. Handlingsteel slag (for example for cooling the steel slag or when washing orsieving the crushed steel slag) often produces, as a waste product, suchhighly alkaline effluent waters, with a high content in calcium, sodium,potassium and/or magnesium ions. Bubbling this gas through such effluentwaters will further reduce its carbon dioxide content, whilesimultaneously reducing their alkalinity, reducing the environmentalimpact of both.

Preferably, said slag particles may have sizes not larger than x, said xbeing not larger than 4 mm, preferably not larger than 3 mm, morepreferably not larger than 2 mm, and most preferably not larger than 1mm. Such a small particle size increases the reaction surface andfacilitates the carbonation of the slag particles.

Preferably, said grains may have sizes not smaller than y, said y beingnot smaller than 1 mm, and more preferably not smaller than 2 mm.

A binder, such as cement, may additionally be added during and/or beforesaid aggregation step to help bind the slag particles together into thecoarser grains. This ensures the cohesion of the grains during theirhandling in particular when they are not, or not yet, bound together bya solid matrix containing carbonates formed during the abovementionedcarbonation.

Preferably, said grains may preferably also contain sand, in particularsea sand. Otherwise, carbonation could form a gas-impervious crust ofcarbonate around each grain, thus trapping moisture within each grainwhich could cause the grains to explode during the introduction in thefurnace. By adding such sand to the slag particles during theaggregation step and disrupting the packing of the slag particles, it ispossible to produce granules with increased gas permeability, improvingboth the carbonation of the slag particles at the core of each grain andthe evaporation of moisture from each grain prior to their introductionin the furnace. Moreover, this sand also serves as a source of silicon,and, in the case of sea sand, accelerates the hydration of calcium andmagnesium oxides prior to their carbonation.

Preferably, said grains may also contain carbonaceous particles, suchas, for example coal and/or coke dust. Since the calcination of calciumand magnesium carbonates into calcium, magnesium and carbon oxides is astrongly endothermic reaction, the exothermic combustion of thesecarbonaceous particles will help restore the energy balance in theelectric arc furnace, besides being an additional source of slag foaminggas. Coal and coke dust also have the advantage of being inexpensivebyproducts of, respectively, coal mining and handling and cokeproduction.

Preferably, said grains may also contain bauxite particles. Because theyare a source of aluminium, bauxite particles not only can disrupt thepacking of the slag particles and thus decrease the gas permeability ofthe grains, but also have the additional advantage of improving theretention of halogens, such as fluorine, in the foaming slag after itcools and hardens. Moreover, bauxite having a lower melting point thansea sand, the energy consumption of the electric arc furnace could becomparatively lower.

Preferably, said grains may also contain stone crushing sands and/ordust. Such waste product of quarries, in particular of limestone,dolomitic limestone, dolomite and/or magnesite quarries, besidesdisrupting the packing of the slag particles, can also be an additionalsource of calcium and/or magnesium carbonates at a low cost.

Preferably, said grains may also contain glass particles. Besidesdisrupting the packing of the slag particles, these glass particles,like bauxite, also have the advantage of having a comparatively lowmelting point. Moreover, they provide a means of adjusting thealkalinity of the foaming slag, and can be obtained as a cheap wasteproduct from glass recycling processes.

Preferably, said slag particles may contain a significant amount ofγ-dicalcium silicate, in particular at least 3 wt. %, preferably atleast 5 wt. % and more preferably at least 7 wt. % of γ-dicalciumsilicate. While other uses for solid slag are already known, inparticular in the construction industry, disposal of slag particlescontaining a significant amount of γ-dicalcium silicate has provenparticularly complicated until now, due to their negative properties ofwater absorption. Slag containing γ-dicalcium silicate can absorb largequantities of water.

At ambient temperature, crystalline lime-silicate slag generallycomprises crystals of dicalcium silicate (CaO)₂SiO₂ in both their β andγ polymorphic states. As molten dicalcium silicate slowly cools down andsolidifies, it goes through several polymorphic forms:

α with hexagonal crystal structure,

α_(H)′ with orthorhombic crystal structure,

α_(L)′ with orthorhombic crystal structure,

β with monoclinic crystal structure, and

γ with orthorhombic crystal structure.

As the last transition is linked to an increase of approximately 12% involume, it causes high strains and microcracks in the dicalcium silicatecrystals of the orthorhombic γ polymorphic state. These microcracksexplain the disadvantageous water absorption properties that had beenfound hitherto in slag containing γ-dicalcium silicate, as water isabsorbed by capillarity into them.

The increase in volume in the transition from the β polymorphic state tothe γ polymorphic state not only causes microcracks but even grainfracture and separation. As a result, the fine fraction of the slag willbe disproportionately rich in comparatively soft γ-dicalcium silicate.Due to the abovementioned microcracks and the associated capillarity,this fine fraction of the slag will have a water absorption capacity ofover 20%. Moreover, it can retain this water for longer periods of time.

Advantageously, said slag particles may be from stainless steel slag.Stainless steel slag usually contains substantial amounts of heavymetals such as chromium, in particular chromium VI in the form of CrO₄and Cr₂O₇ and molybdenum, which constitute a significant environmentaland public health problem. By recycling as large a proportion of thisslag back into the furnace, this problem can be significantlyalleviated.

The present invention also relates to a foaming slag former for electricarc furnaces prepared according to the process of the invention, and tothe use of such a foaming slag former in an electric arc furnace. Saidfoaming slag former may be introduced into said electric arc furnacethrough at least one chute and/or at least one trough lance, with theadvantage of ensuring an even distribution of the slag within thefurnace.

When weight percentages are given in the present specification, theseare percentages in dry weight.

A particular embodiment of the invention will now be describedillustratively, but not restrictively, with reference to the followingfigures:

FIG. 1 is a flow chart representing a process for separating a finestainless steel slag fraction for use with a particular embodiment ofthe process of the invention;

FIG. 2 is a diagram representing the phase transitions during thecooling of dicalcium silicate;

FIG. 3 is a schematic diagram representing a particular embodiment ofthe process of the invention; and

FIG. 4 is a schematic diagram of an electric arc furnace wherein afoaming slag former according to the invention may be used.

FIG. 1 illustrates a process for separating a fine fraction of stainlesssteel slag particles. This fine fraction is rich in γ-dicalciumsilicate, and presents water absorption properties that normally preventit being used in mixtures with hydraulic binding agents, such asPortland cement. In this separation process, the molten lime-silicateslag of an electric arc furnace 1 for stainless steel production isemptied in buckets 2, and transported in these buckets 2 to cooling pits3, in which it is left to slowly cool and solidify. As the cooling iscomparatively slow, the slag will not solidify nearly entirely in anamorphous phase, like GBFS (granular material blast furnace slags), butto a large extent in crystalline phases instead. A significant componentof lime-silicate slag is dicalcium silicate (CaO)₂SiO₂. As crystallinedicalcium silicate cools down, it goes through several polymorphic formsas illustrated in FIG. 2:

α with hexagonal crystal structure,

α_(H)′ with orthorhombic crystal structure,

α_(L)′ with orthorhombic crystal structure,

β with monoclinic crystal structure, and

γ with orthorhombic crystal structure.

With pure dicalcium silicate under laboratory conditions, the transitionfrom α_(L)′-dicalcium silicate to β-dicalcium silicate will occur at675° C., then to be followed by the transition from β-dicalcium silicateto γ-dicalcium silicate at 490° C. As the transition from β-dicalciumsilicate to γ-dicalcium silicate involves an increase of 12% in volumedue to their different crystal structure, it will break up the dicalciumsilicate crystals. This pulverizes a fraction of the slag. Thetransition also causes microcracks in the fine γ-dicalcium silicategrains, which appears to explain why this fine dust can absorb andretain large quantities of water. These water absorption properties makethis fine γ-dicalcium silicate dust highly unsuitable for most uses inconstruction.

Since even with the adjunction of chemical stabilisers and othermeasures known to the skilled person, it appears very difficult tocompletely prevent the formation of γ-dicalcium silicate in mainlycrystalline lime-silicate slag, and since in any case these measurescould interfere with the economical operation of the furnace 1, it hasbeen proposed to extract a fine fraction of the slag, since, due to thepulverisation linked to the γ-β transition, this fine fraction isdisproportionately rich in γ-dicalcium silicate.

In the process illustrated in FIG. 1, molten slag is extracted from thestainless steel furnace 1 and brought to cooling pits 3. After cooling,the solidified slag will be dug from these cooling pits 3 and fedthrough a hopper 4. The hopper 4 comprises a grid for stopping alloversized slag pieces 6, in this particular case those bigger than 300mm. As oversized pieces could damage the crushers used in the laterprocess, these oversized pieces 6 are removed for later particulartreatment, such as breaking with hammers and extraction of large metalfragments before being fed again through the hopper 4.

The slag particles smaller than 300 mm fall through the hopper 4 onto afirst conveyor belt. This first conveyor belt then transports themthrough a first metal handpicking cabin 8 to a first crusher 9 and afirst sieve 10. In the metal handpicking cabin 8, operators remove largemetal pieces 11 from the slag particles on the conveyor belt. After theslag particles are crushed in the first crusher 9, they go through thefirst sieve 10 which separates them into three fractions: particlesbigger than 35 mm, particles between 14 and 35 mm and particles smallerthan 14 mm. The fraction of particles bigger than 35 mm is taken by asecond conveyor belt through a second metal handpicking cabin 13 and afirst metal separating magnetic belt 14, where more metal pieces 15 and16 are removed. The particles bigger than 35 mm are then put back intothe first crusher 9. The fraction of particles between 14 and 35 mm goesinto a second crusher 17 and a second sieve 18, where after beingcrushed again it is separated into two fractions: a fraction ofparticles smaller than 14 mm and a fraction of particles bigger than 14mm. The fraction of particles bigger than 14 mm is taken by a thirdconveyor belt through a second metal separating magnetic belt 20, wheremore metal 21 is removed, and back into the second crusher 17.

The fraction of particles smaller than 14 mm from the first sieve 10,and the fraction of particles smaller than 14 mm from the second sieve18 join again and are put together through the third sieve 22, whichseparates them into a fraction 23 of particles smaller than 4 mm and afraction of particles between 4 and 14 mm, this coarser fraction beingsuitable for use, for example, in construction materials.

Within the fraction 23 of particles smaller than 4 mm, a fine fraction24 of particles smaller than 0.5 mm is particularly rich in γ-dicalciumsilicate, and is therefore used in a particular embodiment of theprocess of the invention, illustrated in FIG. 3.

In this process, the particles in said fine stainless steel slagfraction 24 are first aggregated to form a coarser granular material 33with a granulometry between 0 and 4 mm, and then carbonated. However,since the particles in the fine fraction 24 can form large clods duringstorage, in particular in the open, in this particular embodiment, afirst breaking up step is carried out to break up those clods before theaggregation step. For this purpose, this fine fraction 24 is dried, thenfed through a hopper 29 into a rotary harrow 30, and sieved to removeany remaining clods larger than 4 mm, which are then fed back into thehopper 29.

After this breaking up step, the fine fraction 24 is fed into a disc orpan pelletizer 31, in which the slag particles of the fine fraction 24,together with sea sand 32, are aggregated into a coarser granularmaterial 33 by the rotation of an inclined disc or pan around its mainaxis 34. Water 35 is sprayed onto the pelletizer 31 for the aggregationof the slag particles. For this, highly alkaline effluent waters fromthe previous slag treatment steps may be used.

Besides the slag particles of the fine fraction 24, it could also becontemplated to add other materials into this granular material 33,alternatively or in combination to the sea sand 32, for instance acarbonaceous material, such as coal or coke dust, bauxite particles,stone crushing sands and/or dust, glass particles, and/or lime dust. Thegranular material should however contain at least 20 wt. %, preferablyat least 50 wt. % and more preferably at least 75 wt. % of solid slagparticles.

Fresh steel slag usually contains calcium and magnesium oxides, CaO andMgO. For a more complete carbonation of the slag, these oxides can behydrated to convert them into carbonatable calcium and magnesiumhydroxides, Ca(OH)₂ and Mg(OH)₂. In this particular embodiment, the finefraction 24 is stored in the open for some time before the aggregationand carbonation steps, so that at least a partial hydration happensnaturally due to ambient moisture. If the fine steel slag fraction ishowever so fresh that it has not yet been substantially hydrated by theambient moisture, it may be advantageous to also dissolve otheradditives, such as calcium and/or magnesium acetate and/or salts, suchas, in particular, calcium chloride, in the water 34, or to hydrate thewarm slag (under 350° C.) in a steam chamber or autoclave in order toaccelerate this hydration reaction. Tables 1 to 4 show the results ofhydration tests on calcinated (and thus substantially calcium andmagnesium hydroxide-free) samples of the fine fraction 24:

TABLE 1 Hydration with 20 wt. % pure water Hydration time Totalhydroxides [min] Mg(OH)₂ [wt. %] Ca(OH)₂ [wt. %] [wt. %] 43 0.00 0.000.00 236 0.00 0.00 0.00 514 0.08 0.14 0.22 3000 0.11 0.10 0.21

TABLE 2 Hydration with 20 wt. % of an aqu. solution of 0.5 M Mg acetateHydration time Mg(OH)₂ Ca(OH)₂ content Total hydroxides [min] content[wt. %] [wt. %] [wt. %] 105 0.89 0.47 1.20 320 0.78 0.63 1.41 1080 0.730.32 1.23 2653 0.86 0.40 1.26 4379 0.76 0.53 1.30

TABLE 3 Hydration with 20 wt. % of an aqu. solution of 0.5 M Ca acetateHydration time Mg(OH)₂ Ca(OH)₂ content Total hydroxides [min] content[wt. %] [wt. %] [wt. %] 86 1.34 0.08 1.42 163 1.09 0.80 1.89 829 1.071.01 2.08 1276 1.11 0.89 2.00 1914 1.02 0.91 1.93

TABLE 4 Hydration with 20 wt. % of an aqu. solution of 0.5 M CaCl₂Hydration time Mg(OH)₂ Ca(OH)₂ content Total hydroxides [min] content[wt. %] [wt. %] [wt. %] 84 0.00 0.00 0.00 776 0.00 0.61 0.61 1464 0.300.87 1.17 3113 0.16 0.72 0.88

As can be seen from these results, such additives, dissolved in aqueoussolutions can significantly accelerate the hydration of calcium andmagnesium oxides to form hydroxides in the fine steel slag fraction 24.It must also be noted that the sodium chloride present in the sea sand32 also helps accelerate the hydration of the magnesium and/or calciumoxides present in the particles of the fine steel slag fraction 24.

If the rotation speed and inclination of the pelletizer 31 are keptconstant, the grain size of the coarser granular material 33 obtained inthis aggregation step can be roughly controlled by regulating the flowof water 35 and the stay time of the slag particles in the pelletizer31. After being removed from the pelletizer 31, the granular material 33is fed into sieve 36 to remove oversize grains, in this particularembodiment those over 8 mm. Eventually, a fine sieve could also be usedto remove undersize grains, for instance those under 1 mm.

In the next step, this coarser granular material 33 is carbonated, so asto form calcium and magnesium carbonates CaCO₃, MgCO₃ and CaMg(CO₃)₂. Ina particular embodiment, this carbonation step may be carried out in acontinuous manner, for instance within an inclined rotating drum 37 witha flue gas supply 38, and a flue gas exhaust 39. The granular material33 is conveyed by gravity against the flow of flue gas in the drum 37.Said flue gas may be provided by, for instance, an incinerator, a powerplant, a blast furnace or a cement kiln, at substantially atmosphericpressure and a temperature of around 50° C., with about 10% vol. CO₂.The carbonation time can be regulated by the dimensions of the drum 37,its inclination and/or its rotation speed. It has been found that asubstantial level of carbonation may be achieved in as little as 10minutes. Although in this particular embodiment the carbonation iscarried out continuously, alternatively it would also be possible tocarry out batch carbonation instead.

The sea sand 32, and/or other additional particles, through theirdifferent particle size and shape, disrupt the packing of the slagparticles in each grain of the granular material 33, which increases itsgas permeability to, for example, 1·10⁻⁶ m/s, by interconnecting thepores in the grain without significantly increasing the total porevolume. As a result, the carbon dioxide can more easily reach the coreof each grain, contributing to a more complete carbonation. Moreover,this porosity will also ensure that the carbonates, while binding theslag and sand particles together, will not form a continuous, imperviouscrust on the surface of each grain. Since the carbonation reaction isexothermic, the resulting heat will evaporate the internal moisture ofthe granular material 33 at least partially, resulting in a drycarbonated granular material 40 more suitable for use in electric arcfurnaces. If the granular material is still not dry enough directlyafter the carbonation, it may still be left in cold or heated storage todry out more completely. Given the elevated temperature of electric arcfurnaces, at around 1500-1650° C., the introduction therein of agranular material 33 having a moisture content of even as little as 0.2wt. % could result in a sudden and explosive vaporisation of thismoisture, which would be both a safety hazard and highly damaging forthe components of the electric arc furnace, such as its refractorylining. Usually, however, a moisture content under 1 wt. % is consideredsafe.

The flue gas exiting the rotating drum 37 through the flue gas exhaust39 still contains a significant amount of carbon dioxide. Since highlybasic process water, with a pH value which can exceed 11 or even 12, maybe effluent, for instance, from the previous crushing and washing of thesteel slag, this flue gas can still be used to neutralise such effluentprocess water.

Turning now to FIG. 4, the carbonated granular material 40 is suitablefor use as a foaming slag former back in the electric arc furnace 1. Theillustrated electric arc furnace 1 comprises refractory-lined hearth 41and walls 42, a lid 43 and graphite electrodes 44, wherein the lid 43comprises a chute 45, and a trough lance 46 traverses therefractory-lined walls 42. In use, the electric arc furnace 1 contains amelt 47 and slag 48, heated by electric arcs 49. To foam the slag 48,the carbonated granular material 40 may be introduced into the electricarc furnace 1 through the chute 45 and/or blown into the electric arcfurnace 1 through a trough lance 46. Since, at around 1500° C. to 1650°C., the temperature in the electric arc furnace 1 is well above thecalcination temperatures of the calcium and magnesium carbonates in thegranular material 40, these break down into calcium and magnesiumoxides, on one hand, and carbon dioxide, on the other. This carbondioxide forms bubbles in the slag 48, foaming it.

To increase the foam formation, and to at least partially offset theenergy spent in the endothermic calcination of the carbonates in thegranular material, carbonaceous materials such as, for example, coke orcoal, may also be introduced into the electric arc furnace through thechute 45 and/or the lance 46. Moreover, these carbonaceous materials maybe, as described above, incorporated within the grains in the carbonatedgranular material 40, and/or separate from the carbonated granularmaterial 40. In stainless steel production, the introduction ofcarbonaceous materials, acting as reducers, also has the advantage ofpreventing to some extent the formation of chromium oxides, reducing theconsumption of chromium as well as the content of environmentallyproblematic chromium (VI) in the slag 48.

The foamed slag 48 helps protect the refractory lining of the electricarc furnace 1 from electric arc radiation, forms a more efficientthermal blanket over the melt, reducing heat losses, and dampens thenoise from the electric arc furnace 1. Once discharged, this slag 48 maybe reprocessed again according to the processes illustrated in FIGS. 1and 3.

Although the present invention has been described with reference tospecific exemplary embodiments, it will be evident that variousmodifications and changes may be made to these embodiments withoutdeparting from the broader scope of the invention as set forth in theclaims. For instance, the fine slag particles may be left to naturallyhydrate and carbonate, aggregating themselves into a larger block(heap), which is then broken up to produce the granular foaming slagformer. The aggregation step may also be carried out by other means thanthe disc or pan pelletizer described hereabove, such as, for instance, apelletising press. Lime dust, a byproduct of lime production, could alsoadded to the slag particles, both as an inexpensive additional source ofcarbonatable calcium and/or magnesium hydroxides after hydration, and todisrupt the packing of the slag particles in a granular material. Theduration of the carbonation may be adjusted according to thecircumstances, and the carbonation be carried out using differentequipment than the rotating drum described hereabove. Also, theinvention may be applied in the production of steels other thanstainless steel, or even of non-ferrous alloys in which electric arcfurnaces are used. Accordingly, the description and drawings are to beregarded in an illustrative sense rather than a restrictive sense.

The invention claimed is:
 1. Process for preparing a foaming slag former (40) for electric arc furnaces (1), with at least 20 wt. % of solid slag particles, characterised in that it comprises the steps of: aggregating said solid slag particles to form a granular material formed by coarser grains, and carbonating said solid slag particles, after being aggregated in said aggregating step, to produce said slag former (40).
 2. Process according to claim 1, wherein during said carbonating step at least 2 wt. % of carbonates are produced.
 3. Process according to claim 1, wherein said carbonating is carried out with a gas comprising at least 5 vol. % of carbon dioxide.
 4. Process according to claim 3, wherein said gas comprises less than 30 vol. % of carbon dioxide.
 5. Process according to claim 3, wherein said carbonating step is performed at a temperature of between 10 and 100° C.
 6. Process according to claim 3, wherein said carbonating step is performed at a pressure lower than 10 bar.
 7. Process according to claim 3, wherein after said carbonating said gas is used to reduce alkalinity of an effluent water having a pH higher than
 11. 8. Process according to claim 1, wherein said slag particles have sizes not larger than x, said x being not larger than 4 mm.
 9. Process according to claim 1, wherein said coarser grains have sizes not smaller than y, said y being not smaller than 1 mm.
 10. Process according to claim 1, wherein a binder is added during and/or before said aggregating step to help bind the slag particles together into the coarser grains.
 11. Process according to claim 1, wherein said granular material contains sand (32).
 12. Process according to claim 1, wherein said coarser grains contain carbonaceous particles.
 13. Process according to claim 1, wherein said coarser grains contain bauxite particles.
 14. Process according to claim 1, wherein said coarser grains contain stone crushing sands and/or dust.
 15. Process according to claim 1, wherein said coarser grains contain glass particles.
 16. Process according to claim 1, wherein said slag particles contain a significant amount of γ-dicalcium silicate.
 17. Process according to claim 1, wherein said slag particles are from stainless steel slag.
 18. Process according to claim 1, wherein said aggregating step comprises aggregating said solid slag particles into a block and then, after said carbonating step, breaking up said block to produce said granular material.
 19. Process according to claim 1, wherein said slag particles contain at least 3 wt. % of γ-dicalcium silicate.
 20. Foaming slag former (40) for electric arc furnaces (1) prepared according to claim 1, wherein the foaming slag former (40) comprises carbonated solid slag particles aggregated to form a granular material formed by coarser grains.
 21. Method of forming a slag on a melt in an electric arc furnace (1), said method comprising adding a foaming slag former (40) prepared according to claim 1 to said electric arc furnace (1).
 22. Method according to claim 21, wherein said foaming slag former (40) is introduced into said electric arc furnace (1) through at least one chute (45) and/or at least one trough lance (46).
 23. Method according to claim 21, wherein said foaming slag former (40) is introduced into said electric arc furnace (I) concurrently with a carbonaceous material. 